The PVDF/adhesive composition bilayer film must sometimes be folded or stretched before being fixed to the substrate. It must also sometimes be folded or stretched before being placed in a mould into which the substrate is then injected. According to another possibility, the substrate once coated must sometimes be folded. The folding or stretching causes whitening.
The prior art has already described coating substrates with PVDF films, but has never mentioned the problem of whitening.
Patent GB 1 578 517 describes an ABS coated with a PVDF film, a polyurethane layer possibly being placed between the PVDF and the ABS.
U.S. Pat. No. 4,226,904 describes a PMMA covered with a PVDF film. To improve the adhesion, a PMMA solution in dimethylformamide is deposited on the PVDF film and, after the solvent has evaporated, the PVDF film is pressed onto the PMMA.
U.S. Pat. No. 4,415,519 describes an ABS or PVC substrate covered with a PVDF film, an adhesive being placed between the PVDF and the substrate. This adhesive may be either PMMA or a blend of 40% PMMA, 30% PVDF, and 30% ABS by weight, or else a blend consisting of 30% PMMA, 40% of a polyacrylic derivative and 30% ABS by weight.
U.S. Pat. No. 4,364,886 describes an ABS or unsaturated polyester substrate covered with a PVDF film, an adhesive being placed between the PVDF and the substrate. This adhesive is a blend of 30% PMMA, 40% of an acrylic elastomer and 30% ABS by weight.
U.S. Pat. No. 5,242,976 describes a composition coextrudable with PVDF in order to make it adhere to substrates. The composition is a blend of 27 to 50% PMMA, 17.5 to 36.5% PVDF and 25 to 47.45% of an acrylic elastomer by weight.
In all the above documents of the prior art, there is no mentioned of UV stabilisers in the adhesive composition.
U.S. Pat. No. 6,524,686 describes PVDF-coated substrates; the structure comprises, in succession, the substrate, an adhesive layer, a PVDF layer made opaque to UV and to visible radiation and a PVDF layer. The opaque PVDF layer is obtained by adding a product chosen from metal oxides, pigments and benzophenones to the PVDF. The examples illustrate only PVDF filled with 15% by weight of zinc oxide.
U.S. Pat. No. 5,256,472 describes two-layer films comprising a first layer essentially consisting of PVDF and of a minor amount of PMMA and a second layer intended to ensure adhesion to a substrate. This adhesive layer consists by weight of 50 to 95 (preferably 70 to 90) parts of PMMA, 5 to 50 (preferably 10 to 30) parts of PVDF and 0.1 to 15 parts of a UV absorber. It is explained that the presence of the UV absorber is necessary because this adhesive layer is sensitive to radiation and that if no UV absorber is included then the adhesive is destroyed and the layer essentially consisting of PVDF debonds from the substrate. The term “PMMA” denotes methyl methacrylate homopolymers or methyl methacrylate copolymers with a copolymerisable monomer and also blends with an acrylate rubber, but without specifying the proportions of PMMA and of rubber. It is also specified that if the proportion of PMMA is less than 50 parts, the UV stabiliser exudes. That document does not specify the latter, but this exudation impairs the transparency of the films and above all causes debonding.
U.S. Pat. No. 6,811,859 describes a composition coextrudable with PVDF and comprising: 20 to 40 parts of PVDF; 40 to 60 parts of PMMA; 5 to 18 parts of an acrylic elastomer; 1 to 4 parts of a UV absorber, the total making 100 parts. It also describes coextruded films consisting of:                a layer of the above coextrudable composition (also called the adhesive layer) and, directly attached to the latter,        a layer based on PVDF comprising, as main constituents, 50 to 100 parts of PVDF per 50 to 0 parts of PMMA, respectively.        
In the prior art, there has never been any mention of the compromise that there must be in terms of transparency, mechanical properties and above all non-whitening when the object obtained is folded. There has now been found a composition coextrudable with PVDF that is flexible, transparent and does not undergo any whitening when the film is folded or stretched and exhibits an excellent compromise between elongation at break, transparency and UV protection.